Explanation: A squelch circuit's primary purpose is to silence a receiver's speaker when no usable signal is present. When a receiver is not tuned to a strong signal, its audio output is dominated by internal electronic noise, commonly heard as hiss or static. The squelch circuit continuously monitors this audio output. When the **presence** of this noise exceeds a predetermined threshold (set by the squelch control), the circuit activates and mutes the speaker. This prevents the operator from hearing constant background hiss. When a strong, clear signal arrives, it "quiets" the receiver, reducing the noise level below the threshold, and the squelch circuit then opens, allowing the desired audio to pass through. Therefore, it is the *presence* of background noise that causes the squelch circuit to function by silencing the receiver. Options C (received tones) and D (received digital codes) describe selective squelch systems like CTCSS or DCS. While these *use* specific signals to *open* the squelch, the underlying, fundamental squelch operation itself relies on detecting the *presence* of noise.